Rewind requests can report a missing target when usage limits are reached

The rewind_conversation request, which programs driving Claude Code use to take a conversation back to an earlier message, has two changes:

• It accepts a further option, refuse_missing_target_if_usage_limited, and the usage_limited reply can now include target_not_found, saying the message to rewind to was not found.
• When checking for later turns it has not seen, rewind now compares turns by their text, so it refuses fewer rewinds by mistake.